The present invention relates to a prestressing steel material for use in the fabrication of prestressed concrete by post-tensioning, and particularly to a prestressing steel material having a coating layer of microcapsules.
Concrete is preloaded with compressive stresses by applying tension to prestressing steel materials. There are two general methods of prestressing, namely pretensioning which is conducted before the concrete sets and hardens, and post-tensioning performed after the setting and hardening of the concrete.
Post-tensioning may be performed in two different manners. In one method, concrete is bonded to the prestressing steel material by means of mortar; in the other method generally referred to as the unbonding process, the prestressing steel material is positioned close to the concrete but separated therefrom by an intervening flowable material such as grease or asphalt.
The first bonding method is typically implemented as illustrated in FIG. 1: prior to pouring concrete, a sheath made of a thin iron plate is buried in the area where the prestressing steel material is to be positioned, and the prestressing steel material is inserted into the space of the sheath before or after the concrete sets, and the concrete then is prestressed by applying tension to the prestressing steel material. Thereafter, any space left in the sheath is filled with a grout such as mortar which will solidify to provide an integral and strong combination of the concrete and the prestressing steel material.
Grout such as mortar may be effective in protecting the prestressing steel material from corrosion but its primary function is to increase the durability of the member so that it may have sufficient rigidity and strength against bending and shear stresses.
Structural designs used to prevent direct contact between the prestressing steel material and the surrounding prestressed concrete are illustred in FIGS. 2 and 3. The design shown in FIG. 2 can be used for the prestressing steel material having a steel member of any form of a wire, bar or strand. A steel member 1 having a grease coating 7 is sheathed with a PE (polyethylene) tube 8. When the steel member 1 with the PE tube 8 is placed within a concrete section 6, the lubricating effect of the intermediate grease coating 7 reduces the coefficient of friction between the steel member and concrete to as low as between 0.002 and 0.005 m.sup.-1. Because of this low coefficient of friction, the design in FIG. 2 provides great ease in post-tensioning a long steel cable in concrete. However, if the prestressing steel material is of short length, the need for preventing grease leakage from either end of the PE tube presents great difficulty in fabricating and handling the prestressing steel material. Furthermore, steel members having screws or heads at ends are difficult to produce in a continuous fashion.
The steel member 1 shown in FIG. 3, which is encapsulated in asphalt 9, has a lightly greater coefficient of friction than that of the structure shown in FIG. 2. However, this design is extensively used with relatively short prestressing steel materials since it is simple in construction, is lead-free, and provides ease in unbonding the prestressing steel material from the concrete, even if the steel member has screws or heads at end portions.
One problem with the design in FIG. 3 is that the presence of the asphalt (or its equivalent such as a paint) may adversely affect the working environment due to the inclusion therein of a volatile organic solvent. Moreover, the floor may be fouled by the splashing of the asphalt or paint. As another problem, great difficulty is involved in handling the coated prestressing steel material during drying after the coating or positioning within a framework, and separation of the asphalt coating can easily occur unless utmost care is taken in ensuring the desired coating thickness.
Further, according to the construction as shown in FIG. 2, although the sufficient corrosion resistance can be obtained by simply tensioning the prestressing steel material after the setting and hardening of the concrete without additional operations such as grouting, the member is unable to exhibit as high a durability as can be attained by grouting, since the prestressing steel material is fixed merely to the ends of the concrete section.
It is therefore more common to adopt the bonding process, rather than unbonding, if design considerations require sufficient rigidity and strength against bonding and shear stresses. The problem however is that the bonding process including the grouting step involves cumbersome procedures as compared with the unbonding process. For example, the bonding process inevitably involves not only the procurement of the sheath, grout, and fittings to be installed at the ends of the concrete section in preparation for grout injection, but also inventory management and installation of these materials, as well as operations and management of grout injection, and an extension of the working time.
Compared with the bonding method, the unbonding process involving no grouting step is very simple to peform and this simplicity in operation makes the unbonding process most attractive from a practical viewpoint. An advantage resulting from this feature is the small number of factors that might contribute to degraded reliability for the resultant construction.